Original Article JOURNAL OF COMPOSITE MATERIALS Identification of plasticity-controlled creep and fatigue failure mechanisms in transversely loaded unidirectional thermoplastic composites Ozan Erartsın , Martin van Drongelen and Leon E. Govaert Abstract In continuous fiber-reinforced thermoplastics, the macroscopic failure mode in transverse long-term failure is dominated by a brittle crack-growth mechanism. Neat thermoplastic matrices, on the other hand, generally display also a plasticity- controlled mechanism in long-term loading at elevated stress levels and/or temperature. This failure mechanism requires a different approach to lifetime prediction than crack growth; hence, it is important to identify it in the long-term performance of composites. In this study, we demonstrate the presence of the plasticity-controlled failure mechanism in long-term failure of trans- versely loaded unidirectional (UD) thermoplastic composites made of glass/iPP, carbon/PEEK and carbon/PEKK. The main method used is to compare the lifetime in cyclic loading to that in static loading at the same level of maximum stress, where an increase in lifetime is characteristic for plasticity controlled failure, and, vice versa, a decrease is indicative for fatigue crack growth. In addition, the applicability of a lifetime prediction method common to plasticity- controlled failure of neat thermoplastics is evaluated for the composites investigated. The results of this study indicate that the plasticity-controlled failure was present in composites, although the extent to which the effects are present varied depending on the materials investigated. Glass/iPP showed the most explicit evidence of the plasticity-controlled failure over the entire load range experimentally covered. Its long-term failure was delayed with a decrease in the stress ratio and lifetime was predicted well using the principles of plasticity-controlled failure. Keywords Creep, fatigue, long-term, plasticity, crack growth, transverse, thermoplastic composite Introduction Thermoplastic composites are often used in engineering applications where they need to satisfy long lifetime requirements under creep and fatigue loading. Typical lifespans for current or potential applications might range from 15–20 years in the automotive industry to 20–25 years for aircraft and wind turbines. 1,2 Since real- time testing for such long durations is time-consuming and impractical, the need for lifetime prediction models making use of accelerated testing methodologies is vital. For composites under off-axis creep loading, where the failure is matrix-dominated, the time-dependent failure clearly finds its origin in the time-dependent nature of the polymer matrix. Consequently, lifetime prediction methods can be used that are based on time-temperature superposition, time-stress superposi- tion, and rate theory, where temperature and/or stress are used to accelerate the failure. 3–5 In long-term cyclic loading, the estimation of the lifetime of composite laminates is a little more challenging. Lifetime predic- tions are commonly based on S-N curves of Production Technology Group, Faculty of Engineering Technology, University of Twente, The Netherlands Corresponding author: Leon E. Govaert, Production Technology, Faculty of Engineering Technology, University of Twente, 7500AE Enschede, The Netherlands. Email: [email protected] Journal of Composite Materials 0(0) 1–19 ! The Author(s) 2020 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0021998320964252 journals.sagepub.com/home/jcm
Identification of plasticity-controlled creep and fatigue failure mechanisms in transversely loaded unidirectional thermoplastic composites
Jun 16, 2023
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